Grinding machine and method for machining workpieces

ABSTRACT

A grinding machine for machining workpieces comprises a machine bed; a workpiece holder comprising a workpiece spindle; at least one grinding spindle comprising a grinding tool for machining; a set of steady rests spaced apart from each other along an axial longitudinal extension of the workpiece, wherein the steady rests are operable in an engagement state and a disengagement state, and wherein the steady rests of the set of steady rests are arranged to engage in the engagement state a workpiece for support, the workpiece being mounted on the workpiece holder; and a control device that is configured to selectively set the steady rests into the engagement state or the disengagement state in such a way that a change between the engagement state and the disengagement state takes place during the machining of the workpiece.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of International Patent Application PCT/EP2017/051295, filed on Jan. 23, 2017 designating the U.S., which international patent application has been published in German language and claims priority from German patent application 10 2016 101 196.7, filed on Jan. 25, 2016. The entire content of that priority application is fully incorporated by reference herewith.

BACKGROUND

The present disclosure relates to a method for machining shaft-like workpieces, for instance to a method for grinding shaft components for vehicle drives, such as rotor shafts, gear shafts, camshafts, crankshafts or the like.

The disclosure present also relates to a machine tool, for instance a grinding machine, which is suitable for carrying out this method. Eventually, the disclosure also relates to a machine control program for a machine tool, for instance for a grinding machine.

From DE 20 2010 004 788 U1 there is known a milling machining center for complete machining of workpieces, comprising a tool spindle and a machine control, wherein an adapter device for deep hole drills is provided, which can be detachably attached to the tool spindle, wherein the adapter device comprises a base body and at least one steady rest associated with the base body for supporting the deep hole drill, and a drive for moving the steady rest along a feed axis of the tool spindle.

From U.S. Pat. No. 3,103,840 A there is known is a multi-spindle automatic lathe for single workpieces, which are clamped between centers and supported by steady rests having rollers and being arranged on a carrier, and which are switchable with a spindle drum, wherein the steady rests are radially adjustable in the direction towards the workpiece, for which radially movable slides are provided, which can be moved via cams.

From U.S. Pat. No. 5,493,761 A there is known a device for deep rolling of cambers in the crank journal and main bearings of a crankshaft.

From U.S. Pat. No. 5,103,596 A there is known a control device for a cylindrical grinding machine for cylindrical workpieces, comprising a deflection suppression device comprising abutment elements that engage the cylindrical workpiece on its two sides, and comprising a control unit that is connected to the deflection suppression device to adjust a contact force of the abutment elements.

From DE 38 14 038 A1 there is known an external cylindrical grinding machine for crankshafts, comprising an adjustable steady rest for supporting the workpiece to be machined.

From EP 0 724 500 B1 there is known a grinding machine comprising a work table and a grinding wheel in relation to which the work table can be displaced, wherein the work table supports a headstock and a tailstock that is adjustable in terms of distance relative thereto, and an adjustable workpiece support, and wherein at least the headstock or the tailstock, and in addition the workpiece support are adjustable relative to the work table.

From DE 10 2011 102 113 A1 there is known a grinding machine for grinding outer surfaces of a workpiece, for instance for external cylindrical grinding and/or external non-cylindrical grinding, comprising a workpiece holder which is arranged to rotatably support a workpiece and/or to drive it in a controlled rotary manner, comprising at least a first and a second set of grinding spindles, the at least first and second sets of grinding spindles being displaceable between a primary engagement position and a secondary engagement position, wherein the first grinding spindle set comprises at least a first primary grinding spindle and a first secondary grinding spindle, wherein the second grinding spindle set comprises at least a second primary grinding spindle and a second secondary grinding spindle, wherein at least the primary or secondary grinding spindle of the first and/or second grinding spindle set comprises a grinding wheel package which comprises a plurality of substantially rotationally symmetrical abrasive tool surfaces which are stepped relative to one another and/or delimited from one another.

The grinding machine known from DE 10 2011 102 113 A1 is suitable for machining elongated, shaft-like workpieces, for instance. These include, for example, camshafts, gear shafts, crankshafts, drive shafts, and such like. In accordance with at least some embodiments of DE 10 2011 102 113 A1, the workpiece to be machined can be mounted on a workpiece holder, which can also be referred to as a workpiece spindle. Furthermore, the grinding machine has a first spindle unit and a second spindle unit, which can, for example, each comprise a primary spindle and a secondary spindle. This has the effect that different machining operations that require different machining tools (grinding wheels) can be carried out without reclamping or retooling the grinding machine. This can include, for example, rough machining and fine machining.

Another feature of the grinding machine known from DE 10 2011 102 113 A1 is that the two spindle units, which may also be referred to as spindle sets, face each other, wherein grinding tools currently used for machining are facing each other and preferably only have a very small distance from each other. This enables the machining of shaft components with a considerable longitudinal extension (large length-diameter ratio) without the need for a feed movement (in the longitudinal direction of the workpiece). This type of machining can also be referred to as plunge grinding. In other words, the grinding tools can be arranged to adequately reflect the desired contour of the workpiece. Accordingly, a grinding tool can be described as a grinding wheel package. Such a grinding wheel package is composed of a plurality of segments or sections which can have a different shape.

It goes without saying that grinding machines, for instance cylindrical grinding machines, can also be designed differently. It is therefore not absolutely necessary to provide two sets of grinding spindles. It is also conceivable to provide only one set of grinding spindles on which an appropriately designed grinding tool is mounted.

The present disclosure relates in certain embodiments to the machining of shaft-like workpieces. This includes, for example, transmission shafts, camshafts, crankshafts, drive shafts and the like in the field of automotive engineering. The shaft-like workpieces do not necessarily have to be completely rotationally symmetrical. Instead, the workpieces can have eccentrically shaped sections, for example in the form of cams, cheeks or trunnions. In general, the workpieces have a large length-diameter ratio, which can be about 3:1, 5:1 or even 7:1 or more. In this context, the reference diameter can be, for example, a maximum diameter of the workpiece.

Furthermore, the increasing use of electric motors as drives for electric vehicles, vehicles with fuel cells, and/or vehicles with hybrid drives is also creating an increasing demand for shaft components, such as rotor shafts, that are manufactured in a very precise manner.

By way of example, in series production or mass production of components for drive technology, such as motors, electric motors, drive trains, valve trains, gears and the like, preferably short cycle times have to be achieved. However, this may not be at the expense of accuracy or the required quality. The required accuracies and tolerances to be achieved are in the range of a few μm (micrometers) for gear shafts, for example. Cycle times for machining in series production or mass production for a gear shaft are well below 60 s (seconds).

A good trade-off between a preferably short cycle time and optimum accuracy can, for example, be achieved by a multi-stage machining method, which includes, for example, pre-grinding and finish grinding of the workpiece using different grinding tools.

It has been observed that although satisfactory results can be achieved with known grinding machines in terms of cycle time and machining accuracy, further optimization is desired. This applies for instance to the machining of shaft-like workpieces with considerable longitudinal extension.

In view of this, it is an object of the present disclosure to present a method for machining workpieces, for instance a method for grinding workpieces, which enables further optimization of the machining method.

It is a further object of the present disclosure to present a respective method that enables a further increase in accuracy, particularly with given operating parameters, such as a given machining time.

It is a further object of the present disclosure to present a respective method that enables an improvement of the repeatability, so that the overall process quality can be increased.

It is a further object of the present disclosure to present a machine tool, for instance a grinding machine, which is arranged to carry out the machining method.

It is a further object of the present disclosure to present a respective method that can also be implemented with existing machine tools with little effort.

It is a further object of the present disclosure to present a respective method wherein an easy retrofitting or equipment of existing machine tools with control technology is possible to allow the method to be carried out.

It is a further object of the present disclosure to present a machine control program that sets up a machine tool to perform the method.

SUMMARY

In regard of the grinding machine, these and other objects of the present disclosure are achieved by a grinding machine for machining workpieces, comprising:

a machine bed,

a workpiece holder comprising a workpiece spindle,

at least one grinding spindle comprising a grinding tool for machining,

a set of steady rests spaced apart from each other along an axial longitudinal extension of the workpiece,

wherein the steady rests are operable in an engagement state and a disengagement state, and wherein the steady rests of the set of steady rests are arranged to engage in the engagement state a workpiece for support, the workpiece being mounted on the workpiece holder, and

a control device that is configured to selectively set the steady rests into the engagement state or the disengagement state in such a way that a change between the engagement state and the disengagement state takes place during the machining of the workpiece.

In regard of the method, the above and other objects of the present disclosure are achieved by a method for machining shaft-like workpieces, for instance for grinding machining of shaft components for vehicle drives, rotor shafts, the method comprising the following steps:

mounting a workpiece having a shaft-like design on a workpiece spindle,

providing a set of steady rests spaced apart along an axial longitudinal extension of the workpiece, the steady rests being arranged to assume one of an engagement state and a disengagement state, the steady rests engaging the workpiece for support in the engagement state,

providing of at least one grinding spindle having at least one grinding tool, and

machining the workpiece, comprising:

producing an infeed motion to bring the grinding tool into engagement with the workpiece,

producing a relative rotation between the grinding tool and the workpiece, and

controlling the set of steady rests to selectively bring the steady rests into the engagement state or the disengagement state, wherein a change between the engagement state and the disengagement state takes place during machining.

In accordance with the present disclosure, the method namely allows a selective engagement or disengagement of the steady rests on the workpiece. Switching between the engagement state and the disengagement state takes place during the machining of the workpiece, for instance multiple times. It has been observed that in this way the machining accuracy and also the repeatability of the machining can be increased. In contrast to machining approaches wherein the steady rests permanently engage the workpiece, selective activation and deactivation of the steady rests can contribute to an increased production quality. This may be the case even if the other process parameters are not or only insignificantly changed.

In other words, it has been observed that an at least temporary relief of at least one steady rest of the set of steady rests may increase the achievable manufacturing accuracy.

Steady rests as such are generally known in the art. Steady rests are also known as steadies. In the context of the present disclosure, steady rests are generally understood to be support elements that are fixed directly or indirectly to the machine tool or grinding machine. Steady rests are generally used to support long or extra-long workpieces during machining. On the one hand, this may apply to workpieces that are clamped on only one side. However, steady rests are also used for workpieces that are picked up on both sides, for example on a workpiece spindle and on a tailstock. Steady rests counteract static deformations and dynamic deformations.

Features and effects in accordance with the teaching of the present disclosure become apparent, for instance, in the case of shaft-like workpieces with considerable longitudinal extension. This involves rotor shafts, gear shafts, drive shafts, cam shafts or crankshafts, for instance. In certain embodiments, the workpieces have a length-diameter ratio of at least 3:1, of at least 5:1 in further embodiments, and of at least 7:1 in further embodiments.

The steady rests are for instance arranged as active steady rests and can engage the workpiece in their engagement state. This can include, for example, a defined contact force or holding force. In the disengagement state, the steady rests can be lifted off the workpiece. However, it is also conceivable that the contact force or driving force of the steady rests in the disengagement state is significantly reduced compared to the engagement state.

According to at least some embodiments, the above method is for instance suitable for plunge grinding or oblique plunge grinding, wherein the grinding tool acts on the workpiece without axial feed (in the longitudinal direction) in relation thereto. Furthermore, the grinding tool is, in some embodiments, arranged as a segmented tool and comprises several portions that are associated with peripheral portions of the workpiece. This includes exemplary embodiments wherein a grinding tool or two grinding tools are in the engaged state, which have a longitudinal extension corresponding to at least 30%, at least 50% in further embodiments, and at least 70%, in further embodiments, of the longitudinal extension of the workpiece.

During the machining process, at least one steady rest is alternately in the active state and in the inactive state, for example according to a square wave function or a sine wave function. Accordingly, the states can be associated with discrete values or ranges of continuous values.

The method is suitable for the manufacture of shaft components for vehicle drives, for instance rotor shafts, transmission shafts, camshafts or crankshafts.

According to an exemplary embodiment of the method, the set of steady rests comprises at least two individually controllable steady rests, wherein a first steady rest is in the engagement state at least temporarily during machining while a second steady rest is in the disengagement state, and wherein the first steady rest is in the disengagement state at least temporarily during machining while the second steady rest is in the engagement state.

In other words, two or more steady rests are provided, which can be operated alternately or at least at a time offset. According to a further exemplary embodiment, the at least two steady rests are operated alternately in the engagement state and in the disengagement state during the machining of the workpiece. When the first steady rest is in the engagement state, the second steady rest is at least temporarily in the disengagement state, and vice versa. This measure may contribute to a further increase in manufacturing quality.

According to a further embodiment of the method, the at least two steady rests are at least temporarily in the same state when the first steady rest is transferred from the engagement state to the disengagement state and the second steady rest is transferred from the disengagement state to the engagement state. In other words, there may be a temporal overlap in the states of the steady rests. This may for instance apply for the engagement state. However, according to alternative embodiments, this may also apply for the disengagement state.

According to this measure, it can be ensured that at least one steady rest is always in the engagement state. This can prevent excessive deflection of the workpiece, for example.

According to a further embodiment of the method, at least two steady rests of the set of steady rests are operated simultaneously in the engagement state and the disengagement state and switched back and forth between engagement state and disengagement state. This may, for example, apply for a subset of the set of steady rests. It is conceivable that a total of four steady rests are provided, two of which are operated simultaneously in the engagement state and two in the disengagement state. This means that two of the four steady rests can be controlled synchronously. According to an alternative embodiment, however, it is also conceivable that all of the steady rests are operated simultaneously in the engagement state and in the disengagement state.

The optimal control of the set of steady rests can be determined empirically or heuristically, depending on the respective application. It is conceivable, for example, to define the duration and sequence of states for a steady rest or a set of steady rest for a new application (a new type of workpiece) in the first place based on empirical values. Starting from such a base condition, optimizations can be carried out, for instance in terms of the manufacturing accuracy or the reproducibility to be achieved. Similarly, the timing or sequence of activation and deactivation of the individual steady rests can be determined, cf. for illustrative purposes, for example, the ignition sequence of the cylinders of an internal combustion engine.

It has been observed that a significant optimization of the manufacturing result can be achieved with manageable effort based on a variation in the sequence of activation/deactivation and/or the duration of the respective states.

However, it is also conceivable to vary the control of the set of steady rests on the basis of stochastic or quasi-stochastic methods in order to choose an optimum on the basis of a plurality of tests.

According to a further embodiment of the method, the steady rests can also be operated in a transitional state, wherein the transitional state describes an imminent change between the engagement state and the disengagement state. In this way, the transition state can be used to indicate a change of state between the engagement state and the disengagement state, and vice versa. In this way, the machine tool can be informed of the upcoming status change with a certain amount of lead time. This may be used to adjust operating parameters at least for a short time in order to facilitate the change between the engagement state and the disengagement state.

In accordance with a further embodiment of the method, in the disengagement state, the steady rests act on the workpiece with a reduced holding force. In the alternative, the steady rests are lifted off from the workpiece in the disengagement state. Conversely, the steady rests act on the workpiece in a defined manner in the engagement state. This can include for instance a defined holding force and/or contact force.

According to a further embodiment of the method, an infeed force with which the grinding wheel acts on the workpiece is reduced when one of the steady rests is switched between the engagement state and the disengagement state. This can further increase the achievable accuracy. It goes without saying that the infeed force does not necessarily have to be reduced each time when any of the steady rests changes between the engagement state and the disengagement state. However, it has been observed that it may be possible to reduce the corresponding load on the workpiece, which is linked with the infeed force, when at least one of the steady rests is switched.

According to a further refinement of this embodiment, the infeed force is reduced while the steady rest is in a transition state that initiates a change between the engagement state and the disengagement state (and vice versa). In this way, a soft transition between the engagement state and the disengagement state can be made possible. The short-term reduction of the infeed force may relieve the workpiece and lead to a further increase in quality.

It goes without saying that the infeed force does not necessarily have to be reduced to zero. Rather, embodiments are conceivable wherein the infeed force is reduced from a previous level (say 100%) to a reduced level (about 50%, 20% or 10%). The infeed force is usually oriented perpendicular to the longitudinal axis of the workpiece. Deviations may occur during oblique plunge grinding. The infeed force can cause the workpiece to bend. In addition to the infeed force, the self-weight of the workpiece can also cause a deflection. Furthermore, dynamic deformations can be preset during machining. Usually the steady rests are arranged to minimize unwanted workpiece deformations (such as deflection) caused by the workpiece itself (self-weight and dynamic effects during machining) or by the tool (machining force of the grinding wheel and/or infeed force of the grinding wheel).

According to a further embodiment of the method, at least some steady rests of the set of steady rests are switched back and forth several times between the engagement state and the disengagement state during machining, wherein in some embodiments at least two steady rests of the set of steady rests are switched back and forth between the engagement state and the disengagement state.

A signal path of the activity of a steady rest resulting from the change between the engagement state and the disengagement state may basically comprise a binary path, i.e. exactly two discrete values connected by steep flanks. However, it is also conceivable that the change between the engagement state and the disengagement state produces a signal curve that is roughly sinusoidal. This applies for instance if the steady rest can also be operated in the transition state in addition to the engagement state and the disengagement state. It is not absolutely necessary that a discrete, constant value is assigned to the respective states. Rather, a state, e.g. for a sinusoidal signal, may involve a certain range of values of the signal (e.g. engagement state: 90% to 70%, transition state: 70% to 30%, disengagement state: 30% to 0%). It goes without saying that other embodiments and intermediate stages are conceivable.

Overall, a state period can result therefrom that includes the engagement state and the disengagement state and, if any, the transition state. The duration of such a period can range from 0.5 s (seconds) to 2 s, 5 s or even more, for instance.

Generally, the duration of the disengagement state may correspond to the duration of the engagement state. However, it is conceivable that the duration of the engagement state is longer than the duration of the disengagement state.

In the context of the present disclosure there is also presented a grinding machine, for instance a cylindrical grinding machine, the grinding machine comprising:

a machine bed,

a workpiece holder having a workpiece spindle,

at least one grinding spindle having a grinding tool,

a set of steady rests that are spaced apart from each other, wherein the steady rests are operable in an engagement state and a disengagement state, and wherein the steady rests are adapted to support a workpiece that is mounted on the workpiece holder in the engagement state for support, and

a control device adapted to selectively bring the steady rests into the engagement state and the disengagement state, wherein a defined change between the engagement state and the disengagement state takes place during the machining of the workpiece.

The grinding machine is in some embodiments arranged for plunge or oblique plunge grinding. This may involve embodiments wherein the machining of the workpiece does not include a feed movement along the longitudinal axis of the workpiece.

The grinding machine is in some embodiments suitable for carrying out the machining method according to at least one of the aspects described herein. The set of steady rests may comprise about 2, 3, 4 or even more steady rests that are directly or indirectly mounted on the machine bed and spaced apart from one another along the longitudinal extension of the workpiece.

According to an embodiment of the grinding machine, the control device is further arranged to control an infeed drive of the grinding spindle in order to reduce an infeed force of the grinding wheel on the workpiece during a transition of a steady rest between the engagement state and the disengagement state.

According to a further embodiment of the grinding machine, the steady rests and the grinding spindle are arranged on opposite sides of the mounted workpiece, wherein the steady rests provide a force component which counteracts the infeed force and acts on the workpiece, for instance in the engagement state. In this way, a deflection of the workpiece due to the infeed forces can be effectively counteracted.

According to a further embodiment of the grinding machine, at least some steady rests of the set of steady rests are arranged as active steady rests and are provided with at least one steady rest drive or can be coupled thereto in order to bring a respective steady rest selectively into the engagement state or the disengagement state.

By way of example, active steady rests can include gripping arms and the like, which can be moved at least sectionally to effect the change of state. The steady rest drive may be arranged as a hydraulic or pneumatic drive. It is also conceivable to arrange the steady rest drive as an electric, magnetic or electromechanical drive.

According to a further embodiment of the grinding machine, the control device is further arranged to bring the steady rests into a transition state which is passed through when changing between the engagement state and the disengagement state, wherein the grinding spindle is controlled in a defined manner during the transition state, for instance to regulate the infeed force of the grinding wheel onto the workpiece.

For explanatory purposes, a traffic light comprising a red, a green and a yellow signal is referred to. The engagement state corresponds to the green signal, for instance. The disengagement state thus corresponds to the red signal. The transition state corresponds to the yellow signal, which indicates the change between the green signal and the red signal (and vice versa). In other words, there does not necessarily have to be a structural change during the transition state at the steady rest itself or at its steady rest drive. Rather, the transition state announces the imminent change between the engagement state and the disengagement state.

According to a further embodiment of the grinding machine, the at least one grinding spindle comprises at least one grinding wheel arranged as a grinding wheel package, wherein the grinding wheel package comprises a multi-part core that comprises various axial sections that are joined together, and wherein the grinding wheel package comprises a plurality of grinding segments associated with corresponding sections of the workpiece to be machined. The core can be arranged at least in sections as a composite body, for instance as a fiber composite body.

Grinding wheel packages arranged in this way enable grinding wheels with a large axial extension that can simultaneously machine large axial sections of a workpiece. It has been observed that for instance when the workpiece is machined simultaneously in a plunge machining process at several sections, the selective control of the steady rests to effect a transition between the engagement state and the disengagement state leads to a considerable improvement in accuracy.

It is conceivable that the grinding wheel, which is arranged as a grinding wheel package, has a longitudinal extension which essentially corresponds to the longitudinal extension of the workpiece. Accordingly, the workpiece may generally be machined along its entire or almost its entire extension.

According to a further embodiment of the grinding machine, several spindle units are provided, for instance two spindle units, wherein at least one of the spindle units comprises a grinding spindle with a grinding wheel mounted in a cantilever manner, and wherein the spindle units are, in some embodiments, mounted on the machine bed facing one another. The spindle units may be generally referred to as spindle sets. The spindle units may each comprise one or more grinding spindles. The cantilever (unilateral) bearing of the grinding wheel allows two spindle units mounted on the machine bed facing each other to be arranged in such a way that the grinding wheels come very close to one another. Accordingly, the workpiece can be machined simultaneously with both grinding wheels, wherein a considerable portion of the longitudinal extension of the workpiece can be machined.

According to a further embodiment of the grinding machine, at least one spindle unit is provided which comprises two grinding spindles, wherein the two grinding spindles comprise a common travel drive, and wherein the two grinding spindles of the spindle unit are movable alternately into an active position or a passive position with respect to the workpiece, for instance by means of a swivel drive which is associated with the spindle unit.

Accordingly, rough machining and finish machining can be performed with only one spindle unit comprising two grinding spindles. In general, a combination of several machining steps requiring different grinding tools is conceivable.

By way of example, the grinding machine is arranged in such a way that two spindle units are provided, each comprising two grinding spindles, the grinding spindles of the spindles having grinding wheels mounted via cantilever bearings. Each of the spindle units may include a swivel drive to selectively engage the workpiece with one of the two grinding wheels. This configuration allows complete machining or nearly complete machining of the workpiece in just one set-up. This can preferably be done by plunge grinding or oblique plunge grinding without requiring a feed movement along the longitudinal axis of the workpiece.

It is to be understood that the claimed machining method has similar and/or identical exemplary embodiments as the claimed grinding machine, and vice versa, in particular as defined in the dependent claims and as disclosed in the embodiments discussed herein.

The above and other objects of the present disclosure are also achieved by a machine control program having program code that is adapted to cause a control device of a machine tool to perform the steps of the procedure according to one of the aspects mentioned herein when the machine control program is executed on the control device.

A computer program/machine control program may be stored/distributed on a suitable non-transitory medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

It is to be understood that the previously mentioned features and the features mentioned in the following may not only be used in a certain combination, but also in other combinations or as isolated features without leaving the spirit and scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the disclosure are disclosed by the following description of a plurality of exemplary embodiments, with reference to the drawings, wherein:

FIG. 1 is a simplified perspective view of an embodiment of a grinding machine having a steady rest arrangement;

FIG. 2 is a simplified perspective view of an alternative embodiment of a grinding machine with a steady rest arrangement;

FIG. 3 is a schematic, greatly simplified top view of an arrangement comprising a workpiece, a set of steady rests and two grinding tools, which can be controlled via a control device;

FIG. 4 is a partial perspective view of a grinding machine to illustrate an exemplary embodiment of a steady rest;

FIG. 5 shows a state diagram illustrating a course of states of a set of steady rests;

FIG. 6 shows is a state diagram illustrating a course of states of a set of steady rests, wherein the course of the representation according to FIG. 6 differs from the course according to FIG. 5;

FIG. 7 shows a state diagram illustrating a course of states of a set of steady rests, wherein a course of states of an infeed drive is also shown for explanatory purposes;

FIG. 8 shows another state diagram for a temporal comparison of different states, which may be assumed by a first steady rest and a second steady rest;

FIG. 9 shows a schematic block diagram illustrating an embodiment of a method for grinding workpieces; and

FIG. 10 shows a schematic block diagram illustrating partial steps of an embodiment of a method for grinding workpieces.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a schematic perspective view of a machine tool 10 that is arranged as a grinding machine.

The grinding machine 10 comprises a machine bed 12, which may be also referred to as a frame. Guides 16, 18 are provided on the machine bed 12. Components of the grinding machine 10 can be moved along the guides 16, 18 and selectively fixed thereon.

In at least some of the figures shown herein there is provided a coordinate system for illustrative purposes. A longitudinal direction is generally referred to as X. A transverse direction is referred to as Z. A vertical direction is called Y. In this way, three translational axes X, Y and Z are defined. A respective rotation about the axes X, Y, Z defines a corresponding rotation axis. It goes without saying that alternative coordinate systems, such as machine coordinate systems and/or workpiece coordinate systems, are also conceivable for explanatory purposes. The necessary conceptual transformation is easily possible for the skilled person.

It goes without saying that terms such as first element, second element, primary element, and secondary element, which are used in the context of the present disclosure, are provided primarily for differentiation purposes and for instance do not concern qualitative weighting.

The grinding machine 10 further comprises a workpiece holder 20, which is mounted on the guide 16, for instance. The workpiece holder 20 includes a workpiece spindle 22. The workpiece spindle 22 may include a drive to selectively drive a tool. By way of example, the workpiece holder 20 comprises a first workpiece holder 24 and a second workpiece holder 26, between which a workpiece 30 is held. The first workpiece holder 24 may include a clamping device for the workpiece 30. The second workpiece holder 26 is arranged as a tailstock, for instance. The workpiece spindle 22 may also include a C-axis drive 28, which allows a defined rotation of the workpiece 30 about its longitudinal axis. The longitudinal axis of workpiece 30 is parallel to the X axis. The C-axis drive 28 allows a defined rotation orientation of the workpiece 30, which enables the machining of eccentric contours of the workpiece 30. By way of example, cams or the like can be ground in this way.

The workpiece 30 is one of a camshaft, a gear shaft, a drive shaft and a crankshaft, for instance. The workpiece 30 has a considerable longitudinal extension. In this respect, the workpiece 30 may also be described as an elongated workpiece. The workpiece 30 has a shaft-like design. In the exemplary embodiment according to FIG. 1, workpiece 30 is held on both sides at the workpiece holder 20, i.e. between the first workpiece holder 24 and the second workpiece holder 26. It is also generally conceivable to hold the workpiece 30 only on one side, i.e. to dispense with the second workpiece holder 26, which is arranged as a tailstock.

The workpiece 30 may also be a rotor shaft of an electric motor. By way of example, this involves a drive motor for a vehicle. The increasing use of vehicles with alternative drives, such as electric vehicles, hybrid vehicles and the like, results in increased use of such components. The workpiece 30 may generally be also referred to as a shaft component for a vehicle drive.

Workpieces 30, which are arranged as a gear shaft, crankshaft, camshaft or drive shaft, often have a considerable longitudinal extension. This involves a high length-diameter ratio, such as a ratio between length and diameter of the workpiece 30 of at least 3:1, at least 5:1, or at least 7:1. Workpieces 30 that are having such a length are often mounted on both sides at the workpiece holder 20. Furthermore, the workpiece 30 can be supported along a longitudinal extension by a steady rest arrangement 32, which comprises a first steady rest 34 and a second steady rest 36, for instance.

The steady rests 34, 36 are arranged to supportively engage the workpiece 30 in order to counteract deformation or bending of the workpiece 30. The steady rests 34, 36 are arranged as active steady rests, in certain embodiments. This includes embodiments wherein the steady rests 34, 36 are provided with drives and/or may be coupled thereto in order to selectively bring the steady rests 34, 36 into an engagement state or a disengagement state.

At least in accordance with some exemplary embodiments, there is no permanent support by the steady rests 34, 36. Rather, the steady rests 34, 36 are temporarily activated and temporarily deactivated in order to increase the accuracy of the grinding process. This may include a simultaneous actuation of the steady rests 34, 36. Furthermore, this may include a reciprocal actuation of the steady rests 34, 36. The steady rests 34, 36 can be activated in phase or out of phase. Activation in the same phase occurs when several steady rests 34, 36 are in the engagement state or the disengagement state at the same time. An out of phase or phase-shifted activation is present if at least temporarily one steady rest is in the engagement state and another steady rest is simultaneously in the disengagement state.

Exemplary embodiments of the present disclosure relate to the control of steady rest arrangements 32 that comprise several steady rests 34, 36. This is explained in more detail hereinafter.

The actual (material removing) machining of the grinding machine 10 is carried out by spindle units 50, 52, which may also be referred to as spindle sets. The grinding machine 10 comprises at least one spindle unit 50, 52. According to the exemplary embodiment illustrated in FIG. 1, the grinding machine 10 comprises a first spindle unit 50 and a second spindle unit 52. The spindle units 50, 52 each comprise at least one headstock (spindle stock).

The first spindle unit 50 comprises a primary grinding spindle 54 and a secondary grinding spindle 56. The second spindle unit 52 comprises a primary grinding spindle 58 and a secondary grinding spindle 60. A grinding tool 62 is mounted on the grinding spindle 54. A grinding tool 64 is mounted on the grinding spindle 56. A grinding tool 66 is mounted on the grinding spindle 58. A grinding tool 68 is mounted on the grinding spindle 60.

The grinding tools 62, 64, 66, 68 are arranged as grinding wheels, in certain embodiments. At least some of the grinding tools 62, 64, 66, 68 are exemplarily arranged as grinding wheel packages and comprise several segments, which may have different diameters, for instance. The grinding tools 62, 64, 66, 68 are generally rotationally symmetrical, at least on their working surfaces.

The spindle units 50, 52 are arranged in such a way that the grinding tools 62, 64, 66, 68 of spindle units 50, 52 that are used for machining are facing each other. This involves an exemplary embodiment wherein the grinding tools 62, 64, 66, 68 are each mounted in a cantilever manner on the grinding spindles 54, 56, 58, 60 respectively associated therewith.

The spindle unit 50 is mounted on a table 72. The spindle unit 52 is mounted on a table 74. The table 72 may also be referred to as the first table. The table 74 may also be referred to as a second table. The first table 72 comprises a guide base 76 and a carriage 78. The second table 74 has a guide base 80 and a carriage 82. The tables 72, 74 are movably mounted on the guide 18 at the machine bed, for instance. In this way, a movement in the X direction parallel to the longitudinal extension of the workpiece 30 can be effected. The guide base 76, 80 and the carriages 78, 82 of the tables 72, 74 can be moved relative to each other. This enables movement along the Z axis. This movement is also referred to as infeed motion or plunge movement. The tables 72, 74 may also be referred to as cross tables or cross slides.

The first spindle unit 50 also comprises a swivel drive 86 that defines a swivel axis 90. The spindle unit 52 comprises a swivel drive 88 having a swivel axis 92. The swivel drive 86 may also be referred to as the first swivel drive. The swivel drive 88 may also be referred to as the second swivel drive. In FIG. 1, the swivel axes 90, 92 of the swivel drives 86, 88 are oriented parallel to the longitudinal axis of the workpiece 30 and to the X axis, respectively.

The tables 72, 74 and the swivel drives 86, 88 associated therewith may be used to selectively engage the primary grinding spindle 54, 58 or the secondary grinding spindle 56, 60 with the workpiece 30. By way of example, the primary grinding spindles 54, 58 with the grinding tools 62, 66 associated therewith may be used for a first machining stage. Accordingly, the secondary spindles 56, 60 with the grinding tools 64, 68 associated therewith may be used for a second machining stage. The various machining stages may involve, for example, machining different sections of the workpiece 30. However, the various machining stages may also involve rough machining and finish machining.

In FIG. 1 there is also indicated a control device designated by 96 which is used to control the functions and machining processes of the grinding machine 10. This includes for instance a control of travel drives, spindle drives, but also a control of the steady rest arrangement 32.

FIG. 2 illustrates in a simplified perspective view a modified embodiment of a grinding machine 110. The grinding machine 110 is at least basically similar to the grinding machine 10 illustrated in FIG. 1, so that in the following emphasis is given to modified components and features. It goes without saying that components, modules and functions of the grinding machines 10, 110 according to the FIGS. 1 and 2 can be combined with each other and/or transferred from one of the two grinding machines 10, 110 to the other.

The grinding machine 110 is provided with a steady rest arrangement 32, similar to the grinding machine 10 according to FIG. 1. The steady rest arrangement 32 of the grinding machine 110 comprises, for example, four steady rests 34, 36, 38, 40. Such an arrangement is also conceivable for the grinding machine 10 according to FIG. 1. The steady rests 34, 36, 38, 40 are mounted at the guide 16, for instance. The steady rests 34, 36, 38, 40 are axially spaced from one another along the longitudinal extension of the workpiece 30 and directly or indirectly coupled to the machine bed 12.

The grinding machine 110 is further provided with a first spindle unit 150, which includes a first set of grinding spindles. In addition, a second spindle unit 152 is provided, which comprises a second set of grinding spindles. The first spindle unit 150 is associated with a primary grinding spindle 154 and a secondary grinding spindle 156. The second spindle unit 152 is associated with a primary grinding spindle 158 and a secondary grinding spindle 160.

The primary grinding spindle 154 of the first spindle unit 150 supports a grinding tool 162. The secondary grinding spindle 156 of the first spindle unit 150 supports a grinding tool 164. The primary grinding spindle 158 of the second spindle unit 152 supports a grinding tool 166. The secondary grinding spindle 160 of the second spindle unit 152 supports a grinding tool 168. The grinding tools 162, 164, 166, 168 of the spindle units 150, 152 are mounted in a cantilever manner. This allows the grinding tools 162, 164, 166, 168 actually used for machining to be brought very close together so that the workpiece 30 can be machined in an area as large as possible.

The grinding spindles 154, 156, 158, 160 of each spindle unit 150, 152 are arranged facing away from each other. The grinding spindles 154, 156, 158, 160 that are associated with the respective spindle unit 150, 152 may be aligned concentrically to each other. However, a parallel offset between the grinding spindles 154, 156, 158, 160 at the respective spindle unit 150, 152 is also conceivable.

The spindle unit 150 is mounted on a table 172, which comprises a guide base 176 and a carriage 178. The spindle unit 152 is mounted on a table 174, which comprises a guide base 180 and a carriage 182. The tables 172, 174 allow the spindle units 150, 152 with the grinding spindles 154, 156, 158, 160 mounted thereon to be moved in a plane that is spanned by the axes X and Z.

The first spindle unit 150 also includes a swivel drive 186, which defines a swivel axis 190. The second spindle unit 152 also includes a swivel drive 188, which defines a swivel axis 192. The swivel axes 190, 192 are oriented parallel to the Y axis. The swivel axes 190, 192 may also be referred to as so-called B axes. The swivel drives 186, 188 allow on the one hand selective activation or deactivation of the primary grinding spindles 154, 156 or the secondary grinding spindles 156, 160. Further, the swivel drives 186, 188 may also be used to position the grinding tools 162, 164, 166, 168 at an angle to the workpiece 30. In other words, the longitudinal axes of the workpiece 30 and grinding tools 162, 164, 166, 168 may have a defined offset angle to each other in such a position. In this way, conical sections of the workpiece 30 can be machined, for instance.

In FIG. 2, the first spindle unit 150 is shown in an intermediate state, which is present for example when changing between the primary grinding spindle 154 and the secondary grinding spindle 156.

With regard to the basic structural configuration of machine tool 10 according to FIG. 1 and machine tool 110 according to FIG. 2, reference is again made explicitly to DE 10 2011 102 113 A1.

FIG. 3 shows a schematic, greatly simplified view of a workpiece 30 that is mounted between a first workpiece holder 24 and a second workpiece holder 26 in order to be machined by grinding tools 62, 66. Furthermore, in FIG. 3 a steady rest arrangement 32 is indicated, which comprises four steady rests 34, 36, 38, 40.

Grinding tools 62, 66 are exemplarily arranged as grinding wheel packages. For example, the grinding tool 62 comprises different segments 200, 202, 204 which are differently shaped. The grinding tools 62-68, 162-168 described in the context of the present disclosure may be for instance arranged as so-called assembled grinding wheel packages which comprise a core (carrier core) on which segments 200, 202, 204 are accommodated. The core can be arranged as a one-piece or multi-piece component, refer also DE 10 2011 102 113 A1. In this way, on the one hand a sufficiently rigid grinding wheel is provided. Furthermore, the grinding wheel package is weight-optimized and has only a low inertia. In addition, the arrangement as a grinding wheel package allows the replacement of only one of the segments 200, 202, 204, for example in the event of wear or in the event of a defect, which may result in further positive cost effect during operation.

The segments 200, 202, 204 are adapted to corresponding peripheral sections 206, 208, 210 of workpiece 30. Accordingly, the grinding tools 62, 66 are primarily suited for plunge grinding. In other words, no relative movement along the X-direction (longitudinal direction of the workpiece 30) between the workpiece 30 and the grinding tools 62, 66 occurs during machining.

In FIG. 3, block arrows designated by 212, 214 illustrate an infeed or infeed drive of the grinding tools 62, 66 in the direction towards workpiece 30. The infeed takes place along the Z axis (infeed axis). It goes without saying that the infeed can also take place along an axis that is inclined relative to the Z axis. This is for instance the case when grinding at an angle (so-called oblique plunge grinding).

At least some of the steady rests 34, 36, 38, 40 of the steady rest assembly 32 include a holding section 218, a drive 220 and an interface 222 for coupling to the control device 96. The holding section 218 may include holding arms, gripping arms, clamping jaws or the like, for instance. In certain embodiments, the holding section 218 is arranged to hold the workpiece 30 at two or three defined points on the circumference thereof. The drive 220 is arranged to act on at least one element of the holding section 218, in certain embodiments. In this way, the steady rest 34, 36, 38, 40 can be lifted off the workpiece 30. In the alternative, it is conceivable that a holding force or preload force, with which the steady rest 34, 36, 38, 40 acts on the workpiece 30, is significantly reduced. In this way, the steady rest 34, 36, 38, 40 can be brought from an engagement state to a disengagement state, and vice versa.

In certain embodiments, the control unit 96 is coupled to the steady rest assembly 32 and to the infeed drives 212, 214. For example, an infeed force with which the grinding tools 62-68, 162-168 act on the workpiece 30 can be adapted to the current state of the steady rests 34, 36, 38, 40 of the steady rest arrangement 32. Thus it is conceivable to reduce the infeed force at least temporarily when a change of state of at least one of the steady rests 34, 36, 38, 40 takes place.

FIG. 3 further illustrates that the steady rests 34, 36, 38, 40 of the steady rest assembly 32 are, in certain embodiments, arranged on one side of the workpiece 30 that is opposite to the grinding tools 62-68, 162-168. In this way, the steady rests 34, 36, 38, 40 can absorb or counteract the infeed force of the grinding tools 62-68, 162-168.

FIG. 4 illustrates a partial view of a grinding machine that comprises a steady rest 34 that engages a workpiece 30 which is mounted on a workpiece holder 20. The steady rest 34 comprises a base 226 that is mounted on a guide 16 on the machine bed 12 of the grinding machine. The base 226 supports a drive 220 and an interface 222. The drive 220 of the steady rest 34 can be arranged as a fluid drive or as an electromechanical drive.

The steady rest 34 comprises a holding section 218. The holding section 218 of the steady rest 34 includes, for example, a first arm 228 and a second arm 230, which hold a section of the workpiece 30 therebetween. The arms 228, 230 may also be referred to as holding arms, gripping arms or clamping jaws. The drive 220 is arranged to selectively open or close the arms 228, 230. A respective movement of the arms 228, 230 is illustrated in FIG. 4 by double arrows designated by 232, 234. The closed position corresponds to the engagement state. The open position corresponds to the disengagement state. In the alternative, the drive 220 may be arranged to control and selectively change the preload force or contact force of the arms 228, 230 with respect to the workpiece 30.

In this way, the steady rest 34 can be controlled via the control device 96 (see FIG. 3) in a defined manner in order to engage the workpiece 30 for support in an engagement state and to be removed from the workpiece 30 in a disengagement state or at least to engage the workpiece 30 with a significantly reduced holding force.

The steady rest 34 shown in FIG. 4 can also be referred to as an active steady rest. The active steady rest 34 can be operated via the control unit 96 during the machining of the workpiece 30, at least in the engagement state and in the disengagement state.

FIG. 5 illustrates a temporal state course of a steady rest arrangement, which comprises four steady rests, for instance according to the steady rest arrangement 32 illustrated in FIG. 3. A horizontal axis (axis of abscissas) illustrates the course of time (t). The diagram shown in FIG. 5 also includes signal curves designated by 238, 240, 242, 244. The signal path 238 is assigned to a first steady rest. The signal path 240 is assigned to a second steady rest. The signal path 242 is assigned to a third steady rest. The signal path 244 is assigned to a fourth steady rest. The signal curves 238, 240, 242, 244 each comprise two states designated by I and II. State I describes the disengagement state of the respective steady rest, for instance. State II describes the engagement state, for instance.

Overall, FIG. 5 shows that the steady rests may be triggered at different times, according to an exemplary embodiment, so that a phase shift occurs between the signal curves 238, 240, 242, 244. This results in a signal curve wherein always at least one of the steady rests is in the engagement state. It goes without saying that the frequency and/or duration of the states I and II do not necessarily have to be constant. Alternative embodiments are conceivable, for example depending on the actual machining task.

FIG. 6 illustrates an alternative embodiment of a signal course for controlling a steady rest arrangement. Signals indicated by 248, 250, 252, 254 may be assigned to a total of four steady rests. In FIG. 6 the signal curves are phase synchronized. Accordingly, all steady rests are simultaneously in the engagement state or the disengagement state.

It goes without saying that a combination of the signal curves illustrated with reference to FIGS. 5 and 6 is easily conceivable, wherein, for example, individual pairs of the set of steady rests are triggered phase-shifted relative to other steady rests (FIG. 5), and wherein the steady rests within the pairs are triggered phase-synchronized (FIG. 6).

As already explained above, according to an exemplary embodiment, the control of the steady rests is to be coordinated or synchronized with the controlling the infeed drives of the grinding tools. In this way, for example, an infeed force acting on the workpiece can be reduced when a change of state or status occurs on one of the steady rest.

In this context, reference is also made to FIG. 7, which illustrates a temporal state course of two steady rests and compares this with a state course of a signal describing the infeed force. A signal indicated by 258 is assigned to a first steady rest. A signal indicated by 260 is assigned to a second steady rest. A signal indicated by 262 describes the course of the infeed force. As described above, the signal curves 258, 260 associated with the steady rests can describe an engagement state and a disengagement state.

FIG. 7 also illustrates by means of inclined flanks 264, 266 a transition state. Flanks indicated by 264 describe a transition state when changing from disengagement state I to engagement state II. Flanks indicated by 266 describe a transition state when changing from engagement state II to disengagement state I. In the illustration according to FIG. 7, the first steady rest changes from the engagement state to the disengagement state exactly when the second steady rest changes from the disengagement state to the engagement state. However, this is merely to be understood as an exemplary embodiment.

FIG. 7 also shows that the infeed force is reduced when a change of state on one of the steady rests takes place. The relationship between the state of the steady rest and the infeed force illustrated in FIG. 7 can also be provided for the signal curves illustrated in FIG. 5 and FIG. 6.

For illustrative reasons, in FIG. 7 the infeed force indicated by the signal curve 262 is only described qualitatively, wherein a first state is designated by I and a second state by II. As an example, state I can describe a reduced infeed force. This also includes a state wherein the infeed force is zero. Condition II describes a maximum infeed force, for instance. It goes without saying that the change between states I and II does not have to occur abruptly with infinitely steep flanks. Instead, the positioning force can also pass through a transition range when changing between states I and II. Furthermore, it goes without saying that the signal curves illustrated in FIGS. 5 to 7 for the control of the steady rests as a whole can have a smoothed curve. The states I and II do not necessarily have to be assigned discrete values (such as open or not open). Accordingly, the condition curves can also include sinusoidal or similarly smoothed curves. The same applies basically to the signal course of the infeed force.

FIG. 8 shows in a modified representation the different states that the steady rests can assume. By way of example, a block designated by 268 describes a first steady rest and a block designated by 270 describes a second steady rest. The steady rests can assume a total of three states, designated by 272, 276 and 274. By way of example, the state 272 indicates the disengagement state. By way of example, the state 274 describes the engagement state. The state 276 describes a transition state. At the transition between the state 272 and the state 274, a steady rest passes through the transition state. The transition state can be used to indicate the actual change of state of the steady rest. In this way, a time window is provided that allows an adjustment of the infeed force. For purposes of comparison, reference is made to a traffic light in road traffic. The transition state corresponds to the yellow signal, which signals a transition between the red signal and the green signal (and vice versa).

The transition state 276 can also be used when the states 272, 276 are actually to be understood as discrete states. Even if the steady rest can only assume—structurally seen—two states (e.g. OPEN and CLOSED) in terms of the engagement with the workpiece, the transition state permits improved flexibility and a higher variety of functions for the control procedure. In FIG. 8, both signal curves indicated by 268, 270 are alternately in the engagement state and in the disengagement state, wherein the transition state is passed through simultaneously upon the change between the states.

It goes without saying that the process charts illustrated in FIGS. 5 to 8 are only of exemplary nature. Based on the exemplarily illustrated basic configurations, a target configuration can be derived, by way of example, by means of heuristic measures when the sequence, frequency and/or phase offset of the signal progressions are varied, for instance. The temporal portion of the states in a period can also be varied. It also goes without saying that a periodically recurring sequence of states does not necessarily have to be present. Rather, it is possible to deviate from a strictly periodic sequence depending on the current type of application.

FIG. 9 elucidates with reference to a schematically simplified block diagram a method for machining workpieces, for instance for grinding rotor shafts, gear shafts, camshafts, drive shafts or crankshafts. These may generally be referred to as shaft components for vehicle drives.

The method comprises a step S10, wherein a shaft-like workpiece is picked up on a workpiece spindle. The workpiece does not necessarily have to be completely rotationally symmetrical. Workpieces with cams or similar eccentrically shaped contours are also conceivable.

A step S12 comprises providing a set of steady rests spaced apart from one another along an axial longitudinal extension of the workpiece. The steady rests are, in certain embodiments, active steady rests that can assume an engagement state and a disengagement state. In the engagement state, the steady rests support the workpiece to prevent excessive deformations, deflections and the like. In the disengagement state, the steady rests are, for example, spaced away from the workpiece. In the alternative, in the disengagement state, at least a holding force or supporting force with which the steady rests engages the workpiece is reduced. In certain embodiments, the steady rests are coupled with a control device of the grinding machine, which is arranged to selectively control individual steady rests or at least subsets of the steady rests.

A further step S14 relates to the provision of at least one grinding spindle, which is provided with at least one grinding tool, for instance a grinding wheel. The grinding spindle comprising the grinding tool mounted thereon is suited for plunge grinding and/or oblique plunge grinding, in certain embodiments.

A further step S16 follows, which includes the machining of the workpiece. Step S16 may involve substeps S18, S20, S22. Step S18 involves the generation of an infeed motion to bring the grinding tool into engagement with the workpiece. Step S20 involves the generation of a relative rotation between the grinding tool and the workpiece, by means of which the actual material removal machining is enabled. The relative rotation between the grinding tool and the workpiece can be achieved by activating a grinding spindle and/or a workpiece spindle.

A further substep S22 of the machining step S16 relates to a control of the set of steady rests, wherein at least some steady rests of the set of steady rests are selectively put into the engagement state or the disengagement state. This is performed while the workpiece is being machined. This selective control of the steady rests during machining may result in a higher manufacturing accuracy.

FIG. 10 illustrates with reference to a schematic, simplified block diagram an exemplary embodiment of a method for controlling a set of steady rests during machining, for instance grinding machining, of a workpiece.

The method comprises a step S50, which may basically be based on the machining step S16 illustrated in FIG. 9. The step S50 comprises various substeps S52-S66, which concern, for example, the control of a first steady rest (steps S52, S58, S64), a second steady rest (S54, S60, S66) and an infeed drive (S56, S62).

During machining, while at least one grinding tool is in contact with the workpiece, the steady rests can be operated alternately in the engagement state and in the disengagement state. It goes without saying that periods of time are also conceivable wherein both steady rests are in the engagement state or in the disengagement state at the same time. By way of example, the substeps S52 and S54 describe a state wherein the first steady rest is in the disengagement state and the second steady rest is in the engagement state. A change of state of the steady rests can be initiated via a so-called transition state. During the transition state, the substep S56 can follow, which may for instance involve a reduction of the infeed force. Thereafter, the actual (structural) change of state of the steady rests may take place. Accordingly, the substeps S56 may also include a further increase in the infeed force. Subsequent substeps S58, S60 describe a period in which, for example, the first steady rest is in the engagement state and the second steady rest is in the disengagement state.

Again, a transition state may follow, which is used, for example, to initially reduce the infeed force, to induce the actual (structural) change of state of the steady rests and then to increase the infeed force again, refer the substep S62. Again, a period may follow in which the steady rests are operated in their new states, refer the substeps S64 and S66, wherein the first steady rest is in the disengagement state and the second steady rest in the engagement state.

It goes without saying that the state courses exemplarily provided and described herein serve as a starting point for many conceivable procedures. It is essential that a plurality of steady rests is provided, at least some of which can be selectively controlled in order to operate the steady rests alternately in an engagement state and a disengagement state during the machining of the workpiece. So there are active and passive phases for at least one of the steady rests. Depending on a workpiece currently to be machined, for instance depending on its geometry and the selected material, optimizations can be carried out, for instance using heuristic and/or empirical methods. 

What is claimed is:
 1. A grinding machine for machining workpieces, comprising: a machine bed, a workpiece holder comprising a workpiece spindle, at least one grinding spindle comprising a grinding tool for machining, a set of steady rests spaced apart from each other along an axial longitudinal extension of the workpiece, wherein the steady rests are operable in an engagement state and a disengagement state, and wherein the steady rests of the set of steady rests are arranged to engage in the engagement state a workpiece for support, the workpiece being mounted on the workpiece holder, and a control device that is configured to selectively set the steady rests into the engagement state or the disengagement state in such a way that a change between the engagement state and the disengagement state takes place during the machining of the workpiece.
 2. The grinding machine as claimed in claim 1, wherein the set of steady rests comprises at least two steady rests that are individually controllable, wherein, during machining, a first steady rest of the two steady rests is at least temporarily in the engagement state while a second steady rest of the two steady rests is in the disengagement state, and wherein, during machining, the first steady rest is at least temporarily in the disengagement state while the second steady rest is in the engagement state.
 3. The grinding machine as claimed in claim 2, wherein the control device is further configured to selectively operate the at least two steady rests in the engagement state and in the disengagement state during the machining of the workpiece.
 4. The grinding machine as claimed in claim 2, wherein the control device is further configured to operate the at least two steady rests at least temporarily in the same state when the first steady rest is transferred from the engagement state into the disengagement state and the second steady rest is transferred from the disengagement state into the engagement state.
 5. The grinding machine as claimed in claim 2, wherein the control device is further configured to operate at least two steady rests of the set of steady rests simultaneously in the engagement state and in the disengagement state, and wherein the at least two steady rests are toggled between the engagement state and the disengagement state.
 6. The grinding machine as claimed in claim 1, wherein the control device is further configured to operate the steady rests in a transition state, the transition state indicating an imminent change between the engagement state and the disengagement state.
 7. The grinding machine as claimed in claim 1, wherein the control device is further configured to control an infeed drive of the grinding spindle to reduce an infeed force of the grinding tool to the workpiece during a transition of a steady rest between the engagement state and the disengagement state.
 8. The grinding machine as claimed in claim 1, wherein at least some of the steady rests of the set of steady rests are arranged as active steady rests and are provided with at least one steady rest drive or are arranged to be coupled therewith to selectively bring a respective steady rest into the engagement state or the disengagement state.
 9. The grinding machine as claimed in claim 1, wherein the control device is further configured to bring the steady rests into a transition state which is passed through upon a change between the engagement state and the disengagement state, and wherein in the transition state the grinding spindle is controlled to regulate the infeed force of the grinding tool on the workpiece.
 10. The grinding machine as claimed in claim 1, wherein the at least one grinding spindle comprises at least one grinding wheel that is arranged as a grinding wheel package, wherein the grinding wheel package comprises a core which is formed as a multi-part and which comprises various axial sections that are joined together, and wherein the grinding wheel package comprises a plurality of grinding segments that are associated with corresponding sections of the workpiece to be machined.
 11. The grinding machine as claimed in claim 1, comprising two or more spindle units, wherein at least one of the spindle units comprises a grinding spindle having a grinding wheel that is overhung mounted.
 12. The grinding machine as claimed in claim 11, wherein at least one of the two or more spindle units comprises two grinding spindles, wherein the two grinding spindles comprise a common travel drive, and wherein the two grinding spindles of the spindle unit are movable alternately into an active position or a passive position with respect to the workpiece.
 13. The grinding machine as claimed in claim 12, wherein the common travel drive is a rotary drive that is associated with the spindle unit.
 14. The grinding machine as claimed in claim 1, wherein the grinding machine is arranged as a cylindrical grinding machine.
 15. A method for grinding shaft-like workpieces using the grinding machine as claimed in claim 1, the method comprising the following steps: mounting a shaft-like workpiece on the workpiece spindle, machining the workpiece, comprising: producing an infeed motion to bring the grinding tool into engagement with the workpiece, producing a relative rotation between the grinding tool and the workpiece, and controlling the set of steady rests by means of the control device to selectively bring the steady rests into the engagement state or the disengagement state, wherein a change between the engagement state and the disengagement state takes place during machining.
 16. The method as claimed in claim 15, wherein an infeed force by means of which the grinding tool acts on the workpiece is reduced when one of the steady rests is switched between the engagement state and the disengagement state, and wherein the infeed force is reduced while the steady rest is in a transition state that initiates a change between the engagement state and the disengagement state.
 17. The method as claimed in claim 15, wherein at least some steady rests of the set of steady rests are toggled several times between the engagement state and the disengagement state during machining.
 18. The method as claimed in claim 17, wherein at least two steady rests of the set of steady rests are toggled alternately between the engagement state and the disengagement state.
 19. A method for machining shaft-like workpieces, comprising the following steps: mounting a shaft-like workpiece on a workpiece spindle, providing a set of steady rests that are spaced apart from one another along an axial longitudinal extension of the workpiece, the steady rests being arranged to assume one of an engagement state and a disengagement state, the steady rests engaging the workpiece for support in the engagement state, providing at least one grinding spindle comprising at least one grinding tool, and machining the workpiece, comprising: producing an infeed motion to bring the grinding tool into engagement with the workpiece, producing a relative rotation between the grinding tool and the workpiece, and controlling the set of steady rests to selectively bring the steady rests into the engagement state or the disengagement state, wherein a change between the engagement state and the disengagement state takes place during machining.
 20. The method as claimed in claim 19, wherein the set of steady rests comprises at least two steady rests that are individually controllable, wherein, during machining, a first steady rest of the at least two steady rests is at least temporarily in the engagement state while a second steady rest of the at least two steady rests is in the disengagement state, and wherein, during machining, the first steady rest is at least temporarily in the disengagement state while the second steady rest is in the engagement state. 